Re: [RFC PATCH v1 1/2] maccess: fix writing offset in case of fault in strncpy_from_kernel_nofault()

[Yonghong Song <yhs@meta.com>]

On 11/9/22 3:23 AM, Alban Crequy wrote:
> On Tue, 8 Nov 2022 12:38:53 -0800
> Yonghong Song <yhs@meta.com> wrote:
> 
>> On 11/8/22 12:35 PM, Yonghong Song wrote:
>>>
>>>
>>> On 11/8/22 11:52 AM, Francis Laniel wrote:
>>>> From: Alban Crequy <albancrequy@microsoft.com>
>>>>
>>>> If a page fault occurs while copying the first byte, this function
>>>> resets one
>>>> byte before dst.
>>>> As a consequence, an address could be modified and leaded to
>>>> kernel crashes if
>>>> case the modified address was accessed later.
>>>>
>>>> Signed-off-by: Alban Crequy <albancrequy@microsoft.com>
>>>> Tested-by: Francis Laniel <flaniel@linux.microsoft.com>
>>>> ---
>>>>    mm/maccess.c | 2 +-
>>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/mm/maccess.c b/mm/maccess.c
>>>> index 5f4d240f67ec..074f6b086671 100644
>>>> --- a/mm/maccess.c
>>>> +++ b/mm/maccess.c
>>>> @@ -97,7 +97,7 @@ long strncpy_from_kernel_nofault(char *dst,
>>>> const void *unsafe_addr, long count)
>>>>        return src - unsafe_addr;
>>>>    Efault:
>>>>        pagefault_enable();
>>>> -    dst[-1] = '\0';
>>>> +    dst[0] = '\0';
>>>
>>> What if the fault is due to dst, so the above won't work, right?
>>>
>>> The original code should work fine if the first byte copy is
>>> successful. For the first byte copy fault, maybe just to leave it
>>> as is?
>>
>> Okay, the dst is always safe (from func signature), so change looks
>> okay to me. Probably mm people can double check.
> 
> My understanding was that the bpf verifier is supposed to check that the
> dst pointer is safe. But I don't know where it is done, and I don't
> know how it can check that the dst buffer is big enough.

Yes, the verifier ensures the buffer actually has the capacity
for the buffer size. So we are fine here for 'dst' buffer.

> 
>>>    
>>>>        return -EFAULT;
>>>>    }
>>>>
>>>> -- 
>>>> 2.25.1
>>>>   
>>
>